Lectures Fifteen

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Lectures Fifteen SixteenDisplay Devices et al
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Components

• Input device
• tablet/pen, mouse, keyboard, etc.
• Output device
• screen, printer/plotter, slide, film/video, etc.
• Graphics engine
• processor, memory, video control

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Simple Raster Architecture
Peripheral Devices
CPU
System Bus
System Memory
Frame Buffer
Video Control
Display
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CRT Display Device

• CRT - Cathode Ray Tube
• light is produced by impacting phosphor coated
  surface with a beam of electrons
• phosphorescence decays through time depending on
  the persistence of the phosphor
• refresh rate no. of redraws/second
• critical fusion frequency (CFF) minimum refresh
  rate which avoids flicker

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Monochrome CRT Schematic
Horizontal vertical deflection plates
Evacuated tube
Phosphorescence
Phosphor coated surface
Electron gun
Electron beam
Focus mechanism
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Colour CRT

• Colour is formed by combinations of different
  coloured phosphorescence
• closely spaced red, green blue dots
• Shadow-mask CRT
• aligned so that each of 3 electron beams can only
  impact one phosphor group
• Most popular arrangement is delta-delta
• triangular placement of guns phosphors

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Delta-Delta Shadow Mask CRT
Red
Electron guns
Shadow mask
Green
Blue
G
R
B
Electron beams
G
R
B
G
R
B
G
R
B
G
Phosphor dot (triad) coated screen
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Interlaced Video

• Interlacing - a technique used to halve the
  bandwidth from memory to video controller
• display frame in 2 passes (fields)
• odd scanlines only (1,3,5...) in 1st field
• even scanlines only (0,2,4...) in 2nd field
• Refresh rate is effectively halved
• field refresh of 50Hz interlaced has a frame
  refresh rate of 25Hz

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Video Cards

• Issues to consider in specifying a video card
• resolutions supported )
• colour depth )- related
• interlacing / non-interlacing )
• size type of the on-board memory, e.g. 8MB
  SGRAM (static graphics RAM)
• double-buffering
• hardware acceleration, e.g. 3D-AGP
• bus interface, e.g. PCI

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Colour Lookup Tables

• An indirection between the pixel value and the
  displayed colour

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10, 50, 190
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Palette

• n-bit pixel gt 2n simultaneous colour palette
• also means 2n entries in the CLUT
• CLUT permits wider choice of colours for this
  palette depending on the number of output bits
• n output bits also implies 2n potential colours
• Example 8-bit pixel 24-bit output
• 256 simultaneous colours drawn from 16.7 million

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Output bits

• The combination of output bits has important
• implications examples
• 24-bits, 8 for each of RGB gt max of 256 grey
  levels
• 8-bits, 3 for R, 3 for G, 2 for B gt max of 4
  grey levels
• also non-linear 256 colour scale (more yellows)
• 9-bits, 3 for each of RGB gt max of 8 grey
  levels
• but now a linear scale of 512 colours
• Sections of the CLUT are sometimes reserved.